[RISCV] Restrict performANY_EXTENDCombine to prevent an infinite loop.

The sign_extend we insert here can get turned into a zero_extend if
the sign bit is known zero. This can enable a setcc combine that
shrinks compares with zero_extend. This reduces the use count of
the zero_extend allowing other combines to turn it back into an
any_extend.

This restricts the combine to only cases where the result is used
by a CopyToReg. This works for my original motivating case. I
hope the CopyToReg use will prevent any converted extends from
turning back into an any_extend.

Reviewed By: luismarques

Differential Revision: https://reviews.llvm.org/D106754

(cherry picked from commit 54588bcc)